1. Field of the Invention
The present invention relates to decoding device, and more particularly, to a soft decision decoding device and method thereof.
2. Description of the Prior Art
The main difference between soft decision decoders and hard decision decoders is that when a received signal contains a one-bit amount of data, for example, with the received signal value of 1 or −1 representing a 1 or a 0 in said bit, hard decision decoders represent said received signal with only one bit, while soft decision decoders represent said received signal with two or more bits. Because it is inevitable that the communications channel tends to attenuate the received signal, the actual value of the received signal could be 0.8, 0.65, or 0.7 out of the nominal 1.0. Therefore, a slicer added in front of the hard decision decoder is usually desired, so that a received signal value larger than zero is judged as 1, and a received signal value smaller than zero is judged as −1. The soft decision decoder is a lot more complicated. In the front of the soft decision decoder a quantizer for quantizing the value of the received signal into a plurality of levels between 1 and −1 is present. For examples, levels such as −1, −0.8, −0.6, −0.4, −0.2, 0, 0.2, 0.4, 0.6, 0.8, and 1 can be determined. Accordingly, the output of the quantizer has 11 different output levels, and at least 4 bits is required to represent the quantized signal. Because the value of the quantized signal is not limited to just −1 or 1, but with higher resolution, the soft decision decoder can determine the reliability of the received signal according to the level of the quantized signal. In other words, the reliability of the received signal is higher when it is quantized to approach 1 or −1. The reliability of the received signal is lower when it is quantized to approach 0.
Because the values of the received signal may vary according to the communications environment, but the number of levels of the quantizer remains unchanged, the generated quantized signal may fall predominantly at certain specific quantizing levels, with the other levels unused and wasted. As a result, the advantage of the soft decision decoder is substantially undermined.